1. Field of the Invention
This invention is in the field of thermal treatment of alkali-containing pulverent raw material used in the manufacture of cement to achieve minimum heat losses and to produce calcined products of controlled alkali content despite variations in alkali content in the raw material.
2. DESCRIPTION OF THE PRIOR ART
It is known in cement chemistry and cement technology to control the alkali content in cement when such content is unfavorable. Alkali combinations in the cement may appreciably shorten the cement solidification time and bring about a breakdown or reversal of the cement. Too high an alkali content may lead to efflorescence of alkali sulfates in the concrete. Then, too, a high alkali content may result in reaction of the alkali material with additives to cause an alkali expansion of the cement and endanger the constancy of volume of the concrete.
When pulverent raw material to be used in the manufacture of cement is calcined, the alkali chlorides are volatilized quantitatively in the calcining furnace and are conveyed off as vapors or by condensation as a fine mist with the furnace exhaust gas. If the alkali materials in the exhaust gas reach the preheater, they are conveyed back into the calcining furnace in combination with the heated raw material. The furnace exhaust gas is thereby constantly enriched with alkali and the raw material is consistently being increased in its alkali content progressively. Consequently, the pulverent raw material used in the manufacture of cement is appreciably impaired with regard to its fusibility and it may result in a caking or clogging of material within the preheater.
In U.S. Pat. No. 3,235,239 there is described a method for the calcining of alkali-containing pulverent raw material to be used in the manufacture of cement in which the preheating of the pulverent raw material takes place in a preheater with alkali-free exhaust air from the cooler in a separate combustion chamber. The alkalies volatilized in the sintering furnace are partially displaced and partially conveyed into the combustion chamber so that as the operating time increases, an alkali circulation is built up in the installation. With very large cement production installations, however, it is not economical to completely displace or abandon the alkali-containing furnace exhaust gases without making use of their heat content since the cost of cement production particularly with present day high energy costs becomes very high.
In German Pat. No. 1,471,115 there is described a method for the production of a low alkali cement from calcite or limestone containing pulverent raw material in which the raw material is calcined with an alkali free exhaust gas from a cooler in a preheater during addition of fuel and is sintered in a calcining furance. The alkali containing exhaust gases of the sintering furnace are cooled by means of preheating of at least a portion of the raw material. The alkali salts condense through the cooling of the furnace exhaust gases and are removed in a dust removal installation. With this method, a low alkali cement can be produced from alkali containing pulverent raw material without substantial loss of heat, if it is possible to prevent a substantial precipitation of alkali materials in the heat exchanger on the pulverent raw material. However, in practice it is difficult to accomplish this result with high alkali contents, particularly with direct heat exchange.